In wind energy conversion systems, such as in a wind powered generator construction, the rotation of a wind driven rotor will act, usually through a transmission, to drive a generator to generate electrical power or will be otherwise used to produce work.
Wind energy conversion systems that generate alternating current power by direct synchronous connection to the power line, using either a synchronous or induction generator, are designed to operate most efficiently at a substantially constant rotor speed. These systems require very precise and rapid control of blade pitch when the torque developed by the rotor reaches the design limit for the wind system. Substantial difficulty has been experienced in the past in providing adequate torque and speed control for this type of system.
In the past, rotor torque and speed control methods have generally been of three types, (i) fluid or electrically actuated pitch control systems, (ii) mechanical pitch adjusting systems, and (iii) other means of limiting rotor behavior without active pitch control.
Control systems of the first type, of which U.S. Pat. No. 4,160,170 is illustrative, are generally used to control blade pitch for horizontal axis, constant speed rotors and usually function to regulate generated power. These control systems may also have additional functions, as for example, setting the blade pitch for rapid start-up and feathering the blades for rotor shut-down at excessively high wind speeds. While such pitch control systems are readily applied to control of generated power, they suffer from a number of shortcomings which limit their use generally to large and expensive wind systems. Electrically actuated systems are highly complex, requiring wind velocity sensors, averaging circuits and the like, and may have low reliability, slow control rates, and high cost. Generally speaking, control systems having fluid actuation provide more rapid control than electrical actuation, but suffer from either greater complexity or less precise control.
U.S. Pat. Nos. 2,666,149, 4,006,925 and 4,066,911 illustrate various types of mechanical pitch control mechanisms. Mechanical systems of various types have been widely used to provide partial limiting of rotor speed, but do not directly control power and, as such, are not suitable for use with modern constant speed rotors.
A fixed pitch, constant speed rotor is an example of a third type of rotor control. In such devices power output is intrinsically determined by aerodynamic stall of the blades at high wind speed. Shortcomings of this type of control include exceptionally high structural loads, poor power control and potentially undesirable noise and vibration.